As stored program-controlled switching systems have evolved, a wide variety of useful features have been developed to extend the communication capabilities such systems provide. One example is call forwarding where a customer provides a forward-to directory number to the switching system and the switching system thereafter forwards all incoming calls for the customer to the provided directory number. Other examples include call waiting, three-way calling, privacy calling, and automatic callback. Many customers find the activation sequences for such features confusing. For example, to activate call forwarding (FIG. 4), a customer must first go off-hook and dial the access code (*72) for call forwarding, then wait for a recall tone (second dial tone) to dial the forward-to directory number. The customer then receives a confirmation tone indicating that call forwarding has been activated.
The activation sequences for telephone features can be made more user-friendly by including in the telephone station a display with associated control keys much like those widely used for automatic teller machines. The display can be used to guide the customer through the activation sequence. The requirements for the interface between such stations, referred to herein as analog display stations, and a stored program-controlled switching system are specified in Bellcore TR-NWT-001273 issued December 1992. A diagram of the activation sequence for call forwarding is shown in FIG. 5. After going off hook, the station displays a menu of telephone features for selection by the customer. The customer presses the key for call forwarding and the station transmits the access code (*72) for call forwarding to the switching system. The switching system transmits a CPE alerting sequence (CAS) comprising a 50 millisecond burst of the dual tones 2170 Hertz and 2300 Hertz to determine whether the calling station is an analog display station. If it is, the station is muted and the station returns a CPE acknowledgement tone (DTMF "A"). The switching system returns frequency shift keyed (FSK) encoded text which causes the station to display a prompt for the forward-to-DN. The station is then unmuted. The customer then keys in the forward-to-DN and the station transmits the keyed forward-to-DN to the switching system. In response, the switching system again transmits the CPE alerting sequence, the station is muted, and the switching system receives a CPE acknowledgement tone from the station. The switching system transmits to the station FSK encoded text comprising a message confirming that call forwarding is activated.
Although this arrangement provides for user-friendly feature activation, it substantially complicates the switching system software for each telephone feature. It also uses switching system resources inefficiently because the speed of interaction between the switching system and, the station is limited by human inputs such as keying in the forward-to-DN in the call forwarding example.